As many people are aware, four-wheel drives vehicles provide traction often unattainable in two-wheel drive vehicles by delivering power to each of the vehicle's four wheels. Attendant with the added traction provided by four-wheel drive is the added complexity of the drive train required to control and deliver power to all four wheels as opposed to only two wheels. For instance, it is desirable to alter the delivery of power to the front wheels and the rear wheels depending upon whether the vehicle is turning, or is being driven on low traction surfaces such as rain, sand, snow or ice-covered surfaces. The delivery of power between the front wheels and the rear wheels of the vehicle is typically handled by a transfer case mechanism which includes either a mechanically or electronically controlled clutch.
In an all-wheel drive system (A4WD), both the front and rear pairs of wheels are continually in drivable engagement with the transfer case, but substantially all of the torque is transferred to one pair of wheels when no slippage is detected. When slippage is detected, a controller adjusts the duty cycle of an electronically controlled clutch to transfer the torque to the nonslipping wheels. Torque is thus transferred in a mode substantially like a two-wheel drive vehicle until slippage is detected.
The inventors herein have recognized that known transfer case control systems may experience excessive clutch cycling when the vehicle is operated in situations where the wheels experience slippage for an extended period, such as in sand or deep snow. In such an instance, the controller will detect slippage and cycle the clutch to adjust the torque split. This establishes traction, whereafter the controller readjusts the torque split, then detects slippage again. This causes a repeated cycling of the clutch during high torque demand, causing excessive wear of the clutch components. It would be desirable to detect such operating conditions and adjust the duty cycle of the clutch accordingly, without producing adverse effects, such as tire scrubbing on dry pavement.
Additionally, to maximize package space and reduce weight, it is desirable to provide a smaller spare tire in a motor vehicle. However, when a smaller spare tire is provided, the spare may have a smaller rolling radius than the other tires. A wheel having the smaller spare tire installed will therefore rotate at a higher angular velocity. Similarly when a tire has low air pressure, that wheel will have a smaller rolling radius and rotate at a higher angular velocity. In a four wheel drive system which provides a differential for a variation between front to rear velocities, the vehicle driveline controller will detect this higher angular velocity and mistake the higher angular velocity for a "false" wheel slippage. The controller will then transfer torque to the other, "nonslipping", wheels to ensure maximum traction to overcome this "false" slippage situation. The torque transfer to the "nonslipping" wheels causes the differential to cycle excessively and may create a situation where the driveline binds up, creating unacceptable drivability issues as well as potential driveline damage. These concerns are particularly evident in full time four wheel drive systems, where the operator of the vehicle cannot select two wheel drive when a spare tire used.
Strategies designed to compensate for this slip must try to satisfy conflicting vehicle requirements, namely, the need to provide torque to a wheel pair and the need to release the limited slip device to prevent bindup. Rapid release of the limited slip device to prevent bindup may lead to an extended cycling condition, which in turn may produce NVH problems and durability concerns in the limited slip device.